Look at post #50. OK, both the Welcome Song and Hop Skip and Jump are short songs.
As for how: practice. And it helps that I know the ins and outs of each song, spending an average of 2 hours on them.
There are other, non-pony songs which I didn't make that I have extremely high scores on. I have over 9900 for both Tears For Fears - Mad World and Meat Loaf - I would do anything for love on Easy.
Also, I'm a maximalist. I don't post songs until I can score at least 9500 points. That way, I know it can't be too far off.